Here is the new descendant from Berg's excellent gear ranking which organizes tanking gear for WotLK (and 3.0 I would assume). There are some caveats included as much of the gear stats and game mechanics can still change before they go live. The main plus is that the author has included a calculation for the benefits of Expertise which the old ranking system did not include.

One good point of interest, is that much of the endgame raiding gear we currently have will have some good longevity in WotLK. Defense is going to be more difficult to maintain, so gemming for +DEF may become more attactive at some points in gear progression.

At the brink of danger I stand. Clad in armour, shield and sword in hand.

All of the rating conversion ratios are scaling very heavily upwards from level 70 to level 80, so Defense along with all other stats will take a big hit if you don't continue to evolve your gear.

I estimated in my other thread that the conversion ratio is 2.1 times higher at 80 than at 70, meaning that a tank who is precisely at the Defense cap (490) at level 70 will have 467/540 Defense in the same gear at level 80, giving him a 2.93% chance to be critted. This is, of course, for +3 level mobs, something you won't usually have to fight until endgame unless you attempt dungeons that are much higher than your level.

Here's a table illustrating the effects of Defense Rating at each level on the progression to 80.

What is apparent from a quick reading of this table is that you need to add 36-37 Defense Rating at each level from 71-80 to remain capped, and that your crit immunity will disappear very quickly even for same-level enemies. Rating scaling will be the biggest reason why you'd choose an upgrade item over something you already have.

Note: My calculations are based on the assumptions that the RatingBuster figures published by MMO-Champion for level 70 gear are accurate and that the stat conversion ratios will scale linearly from level 70 to 80.